1. Field of the Invention
The present invention relates to a toner for developing electrostatic image to be used in developing an electrostatic image formed by an electrophotographic method or an electrostatic recording method with a developer and a resin particle dispersion solution for a toner for developing electrostatic image.
2. Description of the Related Art
Today, a method such as an electrophotographic method for visualizing image information via an electrostatic image has been employed in various fields. In the electrophotographic method, an electrostatic image is formed on a photoconductor by charging and exposure steps, and the electrostatic latent image is developed by a developer containing a toner for developing electrostatic image (hereinafter, referred to as a “toner” in some cases) and is visualized by transfer and fixation steps. The developer to be used in this case includes a two-component type developer composed of a toner and a carrier and a single-component type developer using a magnetic toner or a non-magnetic toner alone, and a production method of the toners is generally a kneading and milling production method carried out by melting and kneading a thermoplastic resin with a pigment, a charge control agent, and a releasing agent such as a wax, cooling the kneaded mixture, finely milling the mixture thereafter, and further classifying the milled powder. For the toners, if necessary, inorganic or organic particles for improving the fluidity and cleaning property may be added to the toner particle surfaces.
Recently, copying machines, printers employing color electrophotography and composite machines combining these and a facsimile machine have surprisingly been spread, but in the case where proper gloss in color image reproduction and high transparency to obtain excellent OHP images are to be accomplished, it is generally difficult to use a releasing agent such as a wax. Therefore, a large quantity of oil is applied to a fixing roll for assisting separation, and this leads to a result such that duplicated images including OHP have a sticky feeling and subsequent writing on the images with a pen or the like is difficult and also frequently causes uneven gloss. Waxes such as polyethylene, polypropylene, and paraffins to be used generally in common black-and-white copies are more difficult to use since they deteriorate OHP transparency.
On the other hand, even if the transparency is sacrificed, it is difficult to suppress exposure of waxes to the surface of the toners in the conventional toner production method employing the kneading and milling production method, and therefore, problems such as considerable fluidity deterioration and filming on a developing apparatus and a photoconductor are caused in the case where the toners are used as a developer.
As a method for rationally improving these problems, a production method is proposed employing a polymerization method carried out by dispersing an oil phase consisting of monomers as raw materials of a resin and a coloring agent in a water phase and directly polymerizing the monomers to obtain a toner, whereby waxes are enclosed in the toner and exposure of the waxes to the surface of the toner is suppressed.
Also, as means capable of intentionally controlling the toner shape and surface structure, a production method of a toner by an emulsion polymerization coagulation method is proposed (see, for example, Japanese Patent Application Laid-Open (JP-A) Nos. 63-282752 and 6-250439). These methods are production methods for obtaining a toner by producing a resin dispersion solution, generally by emulsion polymerization, and also producing a coloring agent dispersion solution containing a coloring agent dispersed in a solvent, mixing them, forming agglomerates corresponding to the toner particle diameter, and melting and uniting them by heating.
These production methods not only enclose waxes but also make the toners have a small diameter and thereby make clear and high-resolution image reproduction possible. However, to provide high quality images in the above-mentioned electrophotographic process and maintain stable properties of the toners under various mechanical stresses, it is very important to optimize the selection and the amounts of pigments and releasing agents, suppress exposure of the releasing agent to the surface, as well as improve gloss and releasing property in a state in which no fixing oil present and suppress hot offset by optimization of the resin characteristics.
On the other hand, to reduce energy consumption, techniques of fixation at lower temperature are desired, and particularly in recent years, to thoroughly save energy, it is desired to stop energization of a fixing apparatus at all times other than during use. Accordingly, it is necessary that the temperature of the fixation member of the fixation apparatus is increased instantaneously to the use temperature as soon as electricity is applied. Therefore, it is desirable to lessen the thermal capacity of the fixation member as much as possible, but in this case, the fluctuation amplitude of the temperature of the fixation member tends to become more significant than ever. That is, the overshoot of the temperature after starting the electric application becomes significant, and on the other hand, the temperature decrease owing to feeding of paper becomes significant. Also, in the case where paper with a width narrower than the width of the fixation member is continuously fed, the temperature difference becomes large between a paper-passing part and a paper-non-passing part. Particularly, in the case of using a toner for a high speed copying machine or a printer, the electric power capacity tends to be insufficient, and thus, the above-mentioned phenomenon tends to be caused easily. Accordingly, an electrophotographic toner which is to be fixed at a low temperature, causes no offset up to a high temperature range, and has a wide range of so-called fixation latitude has been desired strongly.
It is known that, as means for lowering a fixation temperature of a toner, a crystalline resin obtained by condensation polymerization and showing sharp melting behavior depending on temperature (hereinafter, a resin obtained by condensation polymerization is referred to as a condensation polymerization type resin) is employed as a binder resin composing a toner. However, the crystalline resin is difficult to crush by a melting, kneading, and milling method and therefore is, in general, not usable in many cases. Further, to polymerize the condensation polymerization type resin, reaction at a high temperature exceeding 200° C. and a considerably decreased pressure for no less than 10 hours under stirring with a high motive force is required, resulting in consumption of a large quantity of energy Therefore, in many cases, a huge investment in equipment is necessary to obtain durable reaction facilities.
On the other hand, in the case of carrying out a toner production method by an emulsion polymerization and coagulation method as described above, after polymerization, a condensation polymerization type crystalline resin may be emulsified to be latex and then coagulated with a pigment and a wax and then melted and united. However, at the time of emulsification of the condensation polymerization resin it is necessary to carry out very inefficient and energy-consuming steps of emulsifying the polymer by high shearing force under a high temperature exceeding 150° C., dissolving the polymer in a solvent, dispersing the obtained solution subjected to treatment for decreasing the viscosity in water, and then removing the solvent.
Meanwhile, it has been found that polymerization is made possible at a temperature of 100° C. or lower by a polymerization catalyst containing a rare earth element such as scandium (see, for example, Macromolecules, 36, 1772-17774 (2003)). However, with respect to the polyesters obtained by polymerization using an innovative polymerization catalyst, although the catalytic chemistry, mechanism, side reactions, and effects of the remaining catalyst are enthusiastically being investigated today, technical investigations regarding which characteristics should be controlled for practical applications have not been carried out sufficiently yet. Consequently, application of the resins to resins for toners have not been investigated sufficiently yet.
There is a report that condensation polymerization of polyesters in a water-based medium is possible (see, for example, U.S. Pat. No. 4,355,154). However, the polymerization mechanism of the technique is unclear with respect to many points, and it is difficult to obtain polymers with high molecular weights, and thus industrial practical application is still far away. Naturally, application of the polymerization technique of the polyesters to toners has not yet been investigated sufficiently, and even if the above-mentioned method is simply employed, it is thoroughly impossible to obtain sufficient strength, chargeability, environmental stability, and high-quality image properties as a toner.
As described above, there is no technique of producing a condensation polymerization type resin with a substantially low environmental load or a technique of applying the condensation polymerization type resin produced in water as a resin for a toner. Further, it is difficult to avoid a problem of hydrolysis at the time of emulsification of the condensation polymerization type resin in water, and not only has it been difficult to increase molecular weight of the resin, but also occurrence of unexpected issues in the material planning has been inevitable.
There has been no means made available to achieve the object of producing a toner containing a condensation polymerization type resin and consequently a toner having a small particle diameter with reduced production energy and cost so as to satisfy the demand of users in recent years for high quality images as printing or copying output.